1. When Oxytricha were isolated singly and in two's into 1 or 2 drops of hay medium, there was no difference in the rate of reproduction during 24 hours. 2. In similar experiments using animals isolated singly and in two's into 5 and 10 drops of hay medium, a higher reproduction-rate was found in the cultures starting with two animals. 3. Hay cultures, varying in volume from 1 to 40 drops and inoculated with single animals, showed the highest rate of reproduction in the 1- and 2-drop cultures and progressively decreasing rates in the 5-, 10-, 20-, and 40-drop cultures. 4. These results were found to be independent of temperature, as far as tested; of variations in the hydrogen-ion concentration of the medium within the range used; and of the use of paraffin-lined culture dishes. 5. Several of the experiments showed that increases in temperature caused increases in the reproduction-rate of the protozoans, according to usual expectations, but that relations mentioned above were not affected. 6. Suspensions of the bacterium Pseudomonas fluorescens in a non-nutritive balanced physiological salt medium were found to be the best type of culture medium tried for Oxytricha. Suspensions of Bacillus subtilis were less helpful, while other pure cultures of bacteria tested did not allow reproduction of Oxytricha. 7. Suspensions of Pseudomonas fluorescens in densities of 80-135 million per cubic centimeter were found to be optimal for maximal reproduction in Oxytricka. There was no difference in the average reproduction-rate of one and two animals for 24 hours in 2 drops of this concentration. 8. When single animals were isolated into 2 drops of medium with bacterial densities of Pseudomonas fluorescens two and four times greater than the optimal density, the rate of reproduction was progressively lower with each increase in density. In these supraoptimal densities, two-animal cultures had a higher reproduction-rate for 24 hours than one-animal cultures. 9. Cultures with both one and four animals, in five times the optimal density of Pseudomonas fluorescens, showed a marked inhibition of reproduction. In some cases the animals died in this density. 10. The rates of reproduction of single animals in 2 and io drops of optimal densities were about the same. The rates of reproduction of single animals in 2 and io drops of medium with bacterial concentrations one-tenth that of optimal concentrations were much less than those in optimal concentration, but the 10-drop cultures have the higher rate of reproduction. 11. These results show that many of the so-called allelocatalytic results obtained by Robertson can be explained on the basis of the ratio existing between the infusorian and the bacterial population.